1. Field of the Invention
The present invention relates to a process for preparing superconducting thin film, and more specifically to an improved MBE (Molecular Beam Epitaxy) process particularly for preparing superconducting thin film formed of an oxide material, which has a clean surface, high crystallinity and excellent superconducting properties.
2. Description of Related Art
Oxide superconductors have been considered to have higher critical temperatures than those of metal superconductors, and therefore to have high possibility of practical use. For example, Y--Ba--Cu--O type oxide superconductor has a critical temperature higher than 80 K and it is reported that Bi--Sr--Ca--Cu--O type oxide superconductor and Tl--Ba--Ca--Cu--O type oxide superconductor have critical temperatures higher than 100 K. The oxide superconductor has a crystalline anisotropy in superconducting characteristics, and in particular, its critical current density is maximum in a direction perpendicular to a c-axis of crystal lattice. Therefore, when the oxide superconductor is used, attention should be paid to the crystalline orientation.
In case of applying the oxide superconductor to superconducting electronics including superconducting devices and superconducting integrated circuits, the oxide superconductor has to be used in the form of a thin film. When the oxide superconductor is in the form of a thin film, the above mentioned crystalline anisotropy of superconducting characteristics becomes more remarkable. In addition, to realize high performance superconducting devices and superconducting integrated circuits, two kind of superconducting current paths through one of which superconducting current flows parallel to the substrate and through the other of which superconducting current flows perpendicular to the substrate are necessary. For example, superconducting current flows through a superconducting electrode in the direction parallel to the substrate, flows through an interlayer superconducting current path in the direction perpendicular to the substrate. Therefore, in case of applying the oxide superconductor to superconducting devices and superconducting integrated circuits, a c-axis oriented oxide superconductor thin film of which critical current density is maximum in the direction parallel to the substrate and an a-axis oriented (or b-axis orientated, abbreviated "a-axis oriented" hereinafter) oxide superconductor thin film of which critical current density is maximum in the direction perpendicular to the substrate should be prepared.
The orientation of the oxide superconductor thin film is determined by the film deposition temperature. The film deposition temperature generally means the temperature of the substrate while the oxide superconductor thin film is deposited. An a-axis oriented oxide superconductor thin film can be deposited at the substrate temperature on the order of 50 to 100.degree. C. lower than that of c-axis oriented oxide superconductor thin film.
In order to manufacture superconducting devices and superconducting integrated circuit of multi-layer structure, it is necessary that an a-axis oriented oxide superconductor thin film is stacked on a c-axis oriented oxide superconductor thin film. When the oxide superconductor thin films are stacked, a clean upper surface and high crystallinity are required of the lower oxide superconductor thin film. If the upper surface of the lower oxide superconductor thin film is not clean and contaminants are deposited or oxide is formed on it, the stacked thin films may become physically and electrically discontinuous or an unnecessary weak link of Josephson junction may be formed at the interface. The discontinuity and weak link have adverse effect on the operations of the superconducting device and the superconducting integrated circuit so that their performance become below the determined values or do not operate at all.
The coherence length of the oxide superconductor is very short, therefore, special attention should be given to the contamination of the upper surface of the lower thin film.
In order to clean the surface of the oxide superconductor thin film, in a prior art, the oxide superconductor thin film is heated under a high vacuum so as to vaporize the contaminants. However, oxygen of the oxide superconductor crystalline is unstable and easily escapes when the oxide superconductor thin film is heated under a high vacuum. If some amount of oxygen escapes from the oxide superconductor crystalline, the critical temperature of the oxide superconductor lowers or the oxide superconductor loses its superconductivity.
Therefore, the oxide superconductor thin film which is used a lower layer of a stacked multi-layer structure is required to have a clean outermost surface, high crystallinity and excellent superconducting characteristics.